In the wet process for making Portland cement, the raw materials include such ingredients such as limestone, dolomite, oyster shells, glass furnace slag and other well known high calcium containing products are mixed with silicious materials including slag, clay, shale or other silica containing ingredients and with various aluminum containing and iron containing compounds. The mixture of raw ingredients, using well known process steps, is ground with the addition of water to prepare an aqueous kiln feed slurry which is screened and then pumped into storage tanks preparatory to further blending with other slurries or feeding to a clinkering kiln. Such slurries usually contain from 30-50% by weight of water.
In the kiln, the aqueous slurry is dried and calcined to form a clinker which is then ground to make Portland cement. The water demand of the finely ground limestone-clay slurry is fairly high because large amounts of water are necessary to realize a fluid, pumpable slurry. This limits the rate at which the slurry can be processed and fed to the kiln and it also requires a larger fuel input to effect drying and calcining any given quantity of solids to the clinker stage.
Many substances with dispersant activity have been tried as agents to decrease the water demand of suspended inorganic solids in various high solids water suspensions or slurries for various applications but have proven relatively ineffective or undesirable in the wet process for making Portland cement. Complex phosphates are undesirable because they hydrolyze at the temperature developed during grinding and in storage and because the residual phosphate has an adverse effect on the properties of the final Portland cement product. Lignosulfonates alone or in combination with inorganic salt dispersants require high addition levels for only marginal improvement and also rapidly loose activity during slurry storage. Many other materials are too expensive to be economically utilized in such operations.
U.S. Pat. No. 4,175,975 teaches that the water demand of the slurry can be significantly reduced if a polyacrylic acid salt having a molecular weight of 2,000-50,000 is used in combination with the alkali metal salts of carbonates, bicarbonates, silicates, oxalates, aluminates and borates and ammonium salts of carbonates, bicarbonates, oxalates and borates which form insoluble salts with calcium. Even so, it is desirable to reduce water demand even further, to be able to use a larger variety of dispersants and to be able to more economically reduce water demand.
It has now been found that further improvements in reducing water demand can economically be achieved if any anionic polymer dispersant is added to the slurry at a basic pH provided that the slurry has been treated with carbon dioxide to bind the free cations forming insoluble carbonates in the slurry. Low cost carbon dioxide sources are readily available at wet process plants, a fact which is highly advantageous to the overall economics of the Portland cement production. Also, in addition to the ability to select among numerous anionic polymer dispersants, the amount of dispersant required to realize any given viscosity is generally less than required in said U.S. Pat. No. 4,175,975 which results in further economic advantages. The combination of the carbon dioxide and anionic polymer dispersant is synergistic to a high degree.